Plasticizers are used in polyvinyl chloride insulation for electrical wiring. The plasticizers are used both as processing aids during handling of the polyvinyl chloride resin formulation and its extrusion onto the electrical wire and also as end-use modifiers where they affect the chemical and/or physical characteristics of the insulation on the final product.
While many types of plasticizers are used in polyvinyl chloride insulation covering on electrical wires and cables, the most widely used plasticizers are esters of polybasic organic acids, especially polybasic aromatic acid esters. These plasticizers are the reaction products of aliphatic alcohols with polybasic aromatic acids, principally phthalic acid and trimellitic acid.
Other dibasic acid ester plasticizers are based upon aliphatic acid esters such as esters of adipic, glutaric, pimelic, azelaic, suberic and sebacic acids. Examples of esters of dibasic aliphatic acids used as plasticizers include dioctyl adipate, particularly di-2-ethylhexyl adipate, di-heptyl, nonyl adipate, and di-isononyl adipate. Other such alphatic acid esters used include dioctyazelate, specifically di-2-ethylhexyl azelate and dioctyl sebacate, specifically di-2-ethylhexyl sebacate. Phosphate plasticizers are sometimes also used in polyvinyl chloride insulation. Examples of such plasticizers are isodecyl diphenyl phosphate, isopropylphenyl diphenyl phosphate, 2-ethylhexyl diphenyl phosphate, and tri-2-ethylhexyl phosphate.
The amount and nature of the plasticizers in polyvinyl chloride formulations are dictated in part by the service temperature classification of the wiring product and the thickness of the polyvinyl chloride insulation. In general, higher service temperature classifications require lower volatility (usually higher molecular weight) plasticizer systems. A similar relationship obtains for the thickness of the polyvinyl chloride insulation. Usually a decrease in the thickness of the polyvinyl chloride insulation dictates the need to use a somewhat lower volatility plasticizer system.
The most widely used of the polybasic aromatic acid esters are dialkyl phthalates, examples of which include di-2-ethylhexyl phthalate (DOP), di-isooctyl phthalate (DIOP), di-isodecyl phthalate (DIDP), di-N-octyl, N-decyl phthalate (DNODP), and di-tridecyl phthalate (DTDP). The aforementioned phthalatic acid based plasticizers are conventionally used in low to moderate temperature applications. However, in high temperature wiring applications, e.g., UL-83 90.degree. C. and 105.degree. C. wiring, it is a conventional practice to use plasticizer systems composed of substantial amounts of trimellitate esters. The trimellitate esters are of low volatility and considered to be more stable to oxidation loss from the polyvinyl chloride resin formulation and thus, more suitable for use in high temperature environments. Trimellitic acid based plasticizers include tri(octyl) trimellitate (TOTM), tri(isooctyl) trimellitate (TIOTM), N-octyl, N-decyl trimellitate and tri(isononyl) trimellitate (TINTM).
It is a conventional practice to provide relatively thin polyvinyl chloride insulation on electrical conductors and to surround the PVC insulation with a jacket. Jacketed conductors such as type-THHN and type-TNWN wiring products are provided with PVC insulation surrounded by a nylon jacket. Depending upon the gauge of the wire, the average thickness of the polyvinyl chloride insulation may range from about 15-20 mils with the thickness of the nylon jacket being about 4-5 mils.
While nylon has usually been the jacketing material of choice in high temperature (90.degree. C. or 105.degree. C.) thin wall-type jacketed wiring products, it is also been proposed to use poly (alkylene terephthalate) for this purpose. A specific jacketing material of this type is poly (butylene terephthalate) available from General Electric Company under the designation VALOX.
The plasticizers used in such jacketed thin wall-type wiring products are, consistent with the protocals described previously, of a relatively low volatility. As described in Touchette, N. W. et al. "Plasticizers" Encyclopedia of Physical Science and Technology, Vol. 10, 1987, plasticizer volatility can be characterized in various ways such as in terms of vapor pressure or percent weight loss from plasticized PVC under controlled conditions. A common expedient for characterizing plasticizer volatility is vapor pressure of the plasticizer at 200.degree. C. Plasticizers conventionally used in jacketed thin wall-type wiring products of the type described previously typically have a vapor pressure at 200.degree. C. of less than 0.2 torr (0.2 millimeters of mercury). For example, a low volatility plasticizer formulation used in jacketed wiring products having a surface temperature classification of at least 90.degree. C., is one containing at least 20 percent of a trimellatate plasticizer, e.g., TOTM in admixture with up to 80 percent of relatively high molecular weight phthalate such as dinundecyl phthalate (DUP). DUP has a vapor pressure of 0.2 torr at 200.degree. C., whereas the trimellatate plasticizer typically will have a 200.degree. C. vapor pressure of about 0.05 torr, resulting in an average vapor pressure for the formulation of less than 0.2 torr, specifically about 0.17 torr.
In addition to containing plasticizers, polyvinyl chloride insulations will normally contain stabilizers to retard degradation of the PVC during processing and during use, antioxidants and fillers. The stabilizers may be either organic, or inorganic, or combinations of both. The antioxidants are added in order to prevent oxidative degradation of the plasticizer and also the polyvinyl chloride resin. The antioxidants normally employed in formulating PVC insulation are sterically hindered phenols. Bisphenol A (BPA) is the most widely used. Bisphenol A normally is employed in PVC formulations in only very small amounts, usually substantially less than one weight percent based on the amount of plasticizer in the formulation. However, U.S. Pat. No. 4,806,425 to Chu-Ba discloses the use of relatively high concentrations of bisphenol A in conjunction with certain phthalate esters in formulating insulation coverings having service temperature classifications of at least 90.degree. C. Here, the bisphenol A is employed in an amount of at least 1.5 weight percent based upon the amount of polybasic aromatic acid ester plasticizing system in the insulating material. Topanol CA can also be used as an antioxidant as described in U.S. Pat. No. 4,806,425. Topanol CA is a substantially stronger antioxidant than bisphenol A and can be employed in an amount of about 1/7th of the amount of bisphenol A to achieve equivalent results.